Method to calculate Google PageRank:

While calculating Google PageRank for a web page, all of its inbound links are taken into account. These are links from within the site (Internal Links) and links from outside the site (External Links). PageRank can be calculated using a simple iterative algorithm. PageRank algorithm is in fact elegantly simple and is calculated as follows:
PR (A) = (1-d) + d (PR (T1)/C (T1) + ... + PR (Tn)/C (Tn))
wher;

• PR (A) is the PageRank of page A,
• PR (Ti) is the PageRank of pages Ti which link to page A,
• C (Ti) is the number of outbound links on page Ti and
• d is a damping factor which can be set between 0 and 1

PageRank helps Google's crawl to give preference to the web page, which has got good PR ranking. In fact, PageRank drives Google's monthly crawl, so that sites with higher PageRank get crawled earlier, faster, and deeper than sites with low PageRank. For a large site with an average-to-low PageRank, this is a major obstacle. If your pages don't get crawled, they won't get indexed. If they don't get indexed in Google, people won't know about them. In short, PageRank is the soul and essence of Google, on both the all-important crawl and the all-important rankings.

www.pagerankings.blogspot.com